This invention relates to the field of transmission line structures, and particularly to such structures for very high voltage uses, requiring very tall poles and very long crossarms. The general field of transmission lines is very thoroughly worked, and travelers observe many lines of various forms extending across the country including highly complicated structural steel towers at one extreme and single wooden poles at the other.
An important consideration in the design of a transmission line structure is that of expense. A quite economical structure has been found to consist of a pair of vertical wooden poles spaced laterally of the right of way: shield wires extending from pair to pair of these poles are secured to their tops, and the conductors of the line itself are suspended on insulators from a crossarm secured to the poles below the shield wires. Where the poles are of any considerable height it has been found indispensable to provide diagonal bracing extending transversely between them. Such a structure is shown in Cofer et al. U.S. Pat. No. 2,606,952.
Some concept of the dimensions of interest here is necessary to a fuller understanding of the problems to be solved. For a 400 kilovolt direct current transmission line, with the shield wires to be 101 feet above the ground, the total length of a pole is about 114 feet, of which over 13 feet are underground. The crossarm length is more than 50 feet, the top of a crossarm is about 79 feet above the ground, and the poles are laterally spaced by nearly 20 feet.
The necessity of diagonal cross bracing in so sizable a wooden structure is quite obvious. Moreover, as is pointed out in the Cofer et al. patent, it is highly undesirable to bore any holes or cut any gains in the poles, for structural reasons and to prevent stray currents and lightning discharges from being conducted to the interior of the poles by metal fasteners in the holes. Clamping arrangements have therefore been devised to secure crossarms, braces, and so forth to the poles. The resulting structure initially may have the desired rigidity, but it requires a great deal of hardware, much initial assembly expense in the field, and continued periodic maintenance after installation. It must not be forgotten also that the poles and crossarms must be transported, often for long distances and frequently by road: this involves the expense, nuisance, and delay of obtaining permits, where the load is of excessive length or weight, and the provision of leading and following flag cars in addition to the actual transport vehicle upon occasion, all of which adds very substantially to the expense of a structure by the time it is installed.
Finally, even if properly designed and braced, the support described above is a statically indeterminate structure, the forces in the poles which result from a wind force at the top, for example, being distributed in a fashion which is modified to a major extent, even to serious damage to the poles, by such apparently minor dimensional changes as often occur due to differential pole settling after installation. Omission of the diagonal braces would cure this problem, but cannot be resorted to because of the structural inadequacy of the unbraced poles.